The regulation requires airlines to display the full cost of all fares, including taxes and fees. Airlines that advertise fares for specific routes must also offer a "reasonable numbers of seats" for those flights.

In addition to the fine issued Thursday, the agency ordered the airline to pay a $100,000 penalty that had been suspended after a previous violation of the same rule.

Southwest Airlines said the latest violation was the result of an error in a television ad. The airline said it never intended to offer the $59 fare for flights to New York, Los Angeles and Chicago.

LM Otero / Associated Press

When would-be fliers tried to book the advertised $59 fares, they found no seats available on the specified routes. Above, a passenger checks in luggage at Dallas Love Field.

When would-be fliers tried to book the advertised $59 fares, they found no seats available on the specified routes. Above, a passenger checks in luggage at Dallas Love Field. (LM Otero / Associated Press)

"As soon as we became aware of our mistake, we pulled all incorrect advertisements off the air," the airline said in a statement.

In the previous violation, Southwest advertised $66 one-way fares from Dallas Love Field to Branson, Mo., last year from March 1 to March 21. However, the Transportation Department found that there were no seats available at the advertised price during the sale period.

The agency fined Southwest $200,000 for that violation but suspended half the fine on the condition that the airline "cease and desist" from violating the same rule for one year.